#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <math.h>
int main()
{
	double a, b, c, p, S;//a b c为三角形边长,p为半周长,S为面积
	printf("a=");
	scanf("%lf", &a);
	printf("b=");
	scanf("%lf", &b);
	printf("c=");
	scanf("%lf", &c);
	p = (a + b + c) / 2;
	S = sqrt(p*(p - a)*(p - b)*(p - c));
	printf("S=%lf\n", S);
	return 0;
}